The GAIA DR2 data release currently uses epoch 2015.5 (column ref_epoch)
for all stars, but this may change in future releases. Thus,
right ascension and declination coordinates may be different than those
in J2000 epoch if you're spot-checking them.

// Secant method solver to find when the a fixed star is at a given azimuth to// an observer at the specified latitude and longitude on the Earth.// It takes a first guess at date1 and adds an hour to that to find// its two starting points.//// This function corrects for refraction.// // This currently doesn't handle the case where the star doesn't pass through// the desired azimuth on that date and may give unpredictable results in// this case or the case where the initial guess is far off.//// desiredAzimuth should be specified in the coordinate system specified by// Meeus, which is angle west of south. To convert from normal true compass// bearings, use (trueAzimuth + 180 degrees) mod circle
starSecantAzimuth[date1, starRA, starDecl, lat, long, desiredAzimuth, temperature = 283 K,
pressure=1010 millibars ] :=
{
date2 = date1 + 5 minutes
[azimuth1, altitude1] = refractedAzimuthAltitude[date1, starRA, starDecl, lat, long, undef, temperature, pressure]
[azimuth2, altitude2] = refractedAzimuthAltitude[date2, starRA, starDecl, lat, long, undef, temperature, pressure]